Field of the Invention
Embodiments of the present invention relate to an active magnetic bearing.
Description of Related Art
An active magnetic bearing of the aforementioned type is known from the document “Magnetic bearings—Theory design and applications to rotation machinery; G. Schweitzer-E. H. Masler; Springer 2009—ISBN 978-3-642-00497-1”. As a first approximation, in particular for moderate rotation speeds of the shaft, the shaft of the rotary machine can be assimilated to a rigid solid in rotation. Such as approximation is however no longer valid for high or even very high rotation speeds of a shaft, for example if the rotary machine is a motor-compressor set used in the field of gas compression. In this case, the shaft undergoes radial motion when it is in rotation, due to the deformation thereof, and can then be assimilated to a flexible solid in rotation. Such an active magnetic bearing used in a motor-compressor set is described in the document EP 2 253 852 A1, for example.
Position sensors are generally used in such an active magnetic bearing, with these sensors measuring the position of the shaft at each instant. In order to reduce the bearing size, and to reduce the level of measurement noise caused by the actuators' electromagnetic field on the sensors, each position sensor is placed at a distance from the actuators. Consequently the position signals delivered by the sensors do not represent the radial motion of the shaft of the motor-compressor set at the actuators, and are liable to falsify the correction, by the actuator control means, of the radial motion of the shaft. This proves to be particularly the case when an actuator is situated close to a first antinode of a natural deformation mode of the shaft and the associated sensor is situated close to a second antinode, in phase opposition with the first antinode. Instead of being reduced, the motion of the shaft is then amplified, which degrades the performance of the active magnetic bearing.